The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the present invention.
Pressure sensitive adhesives (PSAs) are viscoelastic materials that can adhere to a variety of substrates. PSA performance is generally evaluated by tack, peel strength, and shear strength. Tack is a measure of how strongly a PSA can bond to a surface under a short contact time, while peel strength is a measure of how strongly a can bond to a surface by peeling it off the surface. Shear strength is a measure of the ability of a PSA to resist deformation under a shear force. Generally, an increase in shear strength for a PSA causes a corresponding decrease in tack and peel strength. This is due to the fact that in order to increase shear strength, the cohesive strength of the PSA must be increased. However, increasing the cohesive strength tends to reduce the ability of a PSA to deform and flow. Thus, the ability of a PSA to wet the substrate is attenuated and corresponding decreases in tack and peel strength are observed.
PSAs may be classified into three categories, according to their method of preparation. Solvent-based PSA films are made by casting a solution of a polymer, typically in an organic solvent. Hot melt PSA films are prepared by casting melt polymers. Latex-based PSA films are prepared by casting an aqueous latex dispersion prepared by emulsion or suspension polymerization. It is often observed that solvent-based PSAs exhibit better performance than latex-based PSAs, displaying larger shear strengths at similar tack and peel strength levels. The increased shear strengths associated with solvent-based PSAs has been attributed to their continuous gel networks. Thus, despite the environmental implications associated with solvent-based PSAs which often contain volatile organic compounds, solvent-based PSAs find continued use in high performance applications requiring large shear strengths.